1. Technical Field
The present invention relates to ultrasonic sensors that measure a distance to an object to be detected and the like by transmitting/receiving an ultrasonic wave.
2. Background Arts
The ultrasonic sensor is widely used for applications, such as in measuring a distance to an object to be detected and in detecting the presence of an object to be detected, as with a photosensor and the like. In particular, the ultrasonic sensor is used for the detection of an object to be detected, which a photosensor is not good at detecting, such as the detection of a transparent or semi-transparent film, such as an ultralight paper or a master for a stencil printing plate, the detection of the position of a paper, both sides of which are printed in black, or the detection of the liquid level of ink or the like, in the field of printing, for example.
In the ultrasonic sensor, an ultrasonic wave is transmitted toward an object to be detected, via a transmitting horn from a transmission element transmitting the ultrasonic wave, and a reflection wave from the object to be detected is received by a reception element via a receiving horn. At this time, because the transmitting horn and the receiving horn are arranged side by side each facing to the same direction, a diffraction wave generated at an edge of an opening of the transmitting horn may be further diffracted at an edge of an opening of the receiving horn and received by the reception element, thereby reducing the detection accuracy of the object to be detected.
Then, as the countermeasure for the diffraction wave, there have been already proposed a method of arranging a sound absorbing member between the opening portions of both horns (Japanese Patent Application Laid-Open No. 63-25578), and a method of causing the diffraction wave to be diffracted into grooves provided around both openings to serve as a standing wave (Japanese Patent Application Laid-Open No. 11-218572).